The invention relates generally to novel materials, namely polymers made from ionic liquids, referred to herein as poly(ionic liquid)s, and, more specifically, to poly(ionic liquid)s for separating gases, particularly carbon dioxide (CO2).
Global warming resulting from the increased CO2 concentration in the atmosphere due to emissions of CO2 from fossil fuel combustion is becoming one of most important environmental issues. 1,2 Recently, CO2 capture and sequestration are receiving significant attention. For carbon sequestration, because the cost of capture and separation are estimated to make up three-fourths of total costs of ocean or geologic sequestration, it is important to develop new materials and methods to separate and capture CO2 from flue gas3,4,5 and other gas mixtures.
Ionic liquids, which are organic salts that become liquids usually below about 200° C., have attracted attention because of their unique chemical and physical properties and wide application potentials.6-12 Recently, CO2 was found to be remarkably soluble in ionic liquids, and thus ionic liquids have been explored as non-volatile, and reversible absorbents for CO2 separation.13-21 For instance, at 15 bar of CO2 pressure, the CO2 solubility in 1-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6]) is about 23 mol. %.19 The CO2 solubility in the ionic liquids is dependent on their cations and substituents, and especially on their anions.19 For example, fluorine-containing anions (e.g. bis(trifluoromethyl-sulfonyl)imide, Tf2N),19 or cations,22 or amine groups23 tend to increase the CO2 solubility. Ionic liquids have also been impregnated into porous materials for developing supported liquid membranes. Such membranes have high CO2 selectivity and permeance because of the selective sorption of CO2 in ionic liquids.24-26 
We have found that poly(ionic liquid)s, the polymers prepared from ionic liquid monomers, have higher CO2 sorption capacity than room temperature ionic liquids. Most importantly, the CO2 sorption and desorption of the polymers are much faster than those of ionic liquids and the sorption/desorption is completely reversible. These poly(ionic liquid)s are thus promising as sorbent and membrane materials for CO2 separation.